Radiant heater having independent sinuous internested tubes



July 26, 1966 H. L. SMITH, JR

RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Filed Nov. 14, 1963 7 Sheets-Sheet 2 INVENTOR HORACE L. SMITH JR. M:

ATTORNEYS TEMP TEMF.

July 26, 1966 RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Filed Nov. 14, 1965 H. L. SMITH, JR

'7 Sheets-Sheet 5 750F 655F 576F 533 790 F a o F DISTANCE DISTANCE INVENTOR HORACE L. SMITH JR.

BY JM,%M

ATTORNEYS July 26, 1966 H. L. SMITH, JR

RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Filed NOV. 14, 1965 7 Sheets-Sheet 4 o 3 Q Q Om m mm Em Bm ww wwmm 0 0% mm 8 N2 mww 9 mwmm 0mm 0% m omwm Nmw 5w w wmmm Qmm m8 t 0mm mwm m mwmw @Q 2% wmmw 0 gm o @e Em 92m N e 5 N mOmQ INVENTOR HORACE L. SMITH JR.

ATTORNEYJ July 26, 1966 H. L. SMITH, JR

RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Filed Nov. 14, 1963 7 Sheets-Sheet 5 INVENTOR HORACE L. 544/ TH JR ATTORNEYS July 26, 1966 H. L. SMITH, JR

RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Filed Nov. 14, 1963 '7 Sheets-Sheet 6 INVENTOR HORACE Lv SMITH JR ATTORNEYS a III/Vigil, w

July 26, 1966 H. L. SMITH, JR

RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Filed NOV. 14, 1965 '7 Sheets-Sheet '7 fill/IA INVENTOR ATTORNEYS HORACE L. SMITH JR.

I 4 I M United States Patent 3,262,494 RADIANT HEATER HAVING INDEPENDENT SINUOUS INTERNESTED TUBES Horace L. Smith, Jr., Richmond, Va., assignor to Hupp Corporation, Cleveland, Ohio, a corporation of Virginia Filed Nov. 14, 1963, Ser. No. 323,848 17 Claims. (Cl. 165107) This application is a continuation-in-part of U.S. application No. 64,965 filed October 25, 1960, by Horace L. Smith, Jr. for Paper Drying System Apparatus and Method, which is now U.S. Patent No. 3,174,228.

This invention relates to heat exchangers and, more particularly, to heat exchange units of the tubular type and to systems employing such units.

Tubular heat exchangers of various kinds are well known. One kind of tubular heat exchanger (hereinafter referred to by the appellation radiator)consists of a planar array to intercommunic-ating, parallel flow passages formed, for example, by metal tubes. Hot water or steam is circulated through the tubes, heating them to a temperature atwhich the tubes will emit substantial quantities of radiant energy.

Such radiators are used for many different purposes such as, for example, to dry continuously moving webs of sheet material. In this and other applications, however, current available radiators have a number of serious limitations. Because of different physical limitations they can only be made to span relatively short distances; and

they are, therefore, not suitable for installations in which heating of a large area is required as in a large paper manufacturing machine, for example.

Another disadvantage of currently available radiators is that the cross-sectional area of the flow passages is relatively small and the volume of liquid that can be circulated through the now available radiators per unit of time is quite limited. Consequently, the radiant energy output of currently available radiators is not sufiicient for many applications.

A further drawback of currently available radiators is that the materials of which they are fabricated are relatively inefficient emitters of radiant energy. For example, rolled sheet steel, a common radiator material, has an emissivity coefficient ranging from 0.65 to only 0.82 (a perfect emitter has an emissivity coefiicient of 1.0). Because of their inefficiency as emitters of radiant energy, also, currently available radiators are unsuitable for many applications.

Another drawback of currently available radiators is that they emit radiant energy in a non-uniform pattern because there are no emitting surfaces between the radiator tubes. Uneven emission patterns are particularly disadvantageous in many radiator applications. For example, in material drying applications the non-uniform emission pattern may result in streaking of the material.

A further drawback of current available radiators is that they are suitable only for relatively low temperature applications. Water, the most commonly employed heat transfer fluid, has a boiling point of only 212 F. Steam has also been employed as the circulating heat transfer medium but it too has drawbacks. As the steam temperature is increased above 212 F., its pressure rapidly increases to the point where, for both economic and design considerations, it is not feasible to fabricate a radiator which is sufliciently strong to withstand the pressure of the steam circulated through it. In addition, provision 3,262,494 Patented July 26, 1966 vantages of the prior art are not present.

In general, the novel radiators of the present invention,

by which the foregoing object is accomplished, include a.

novel planar array of plural, internested, sinuous tubes providing independent flow passages through which a fluid heat transfer medium is caused to flow in counterflow relationship. These novel radiators can readily be made in any desired size and can be arranged to accommodate substantially higher volume rates of flow than currently available radiators of comparable size. Consequently, the radiators of the present invention may be made larger and may be designed to produce a greater heat energy output than the radia-tors heretofore available.

The radiant surfaces of the novel radiators of the present invention are preferably coated with a highly emissive substance, preferably a crystalline, crypto-crystalline or amorphous ceramic such as very thin fused glass frits, preferably of the order of 0.003 to 0.007 inch thick. Such coatings inherently have low thermal conductivity, but, in my preferred thickness range, I have discovered that they have negligible temperature gradients through the coatings and emissivity coefficients very close to 1.0 and are therefore virtually perfect radiators. In this manner, the efficiency of the radiators of the present invention can i e increased well beyond the maximum efiiciencies attainable in presently available radiators.

Another novel feature of the present invention is the provision of conductive webs between and connected to ad jacent tube runs in the radiator. These webs are radiation emitters and greatly increase the area of the radiant surfaces of the radiators of the present invention over that of a currently available radiator of the same size.

By properly configuring these webs a substantially uniform temperature can be maintained across the entire radiant face of a radiator constructed in accordance with the principles of the present invention. These radiators, therefore, emit radiant energy in a substantially uniform pattern and at a uniform intensity, eliminating the problems arising from the non-uniform emission of radiant energy by conventional radiators.

These "benefits are obtained to a quite appreciable extent by using webs having a uniform cross section. They may be achieved to an even greater extent, however, by using a web with a tapered cross section; i.e., a section which is narrower at its midpoint than at the edges where the web is joined to the tubes. By employing webs of tapered section, the ratio of emitted radiant energy to Weight of plate is increased over that obtained from a web of uniform section. Consequently, fora given amount of radiated energy, a radiator utilizing tapered webs may be made lighter than one using uniformly sectioned webs.

J of the present invention resides in the employment, in large radiators of the type described above, of conductive webs having T cross sectional configurations. By using such webs, even very large radiators may be made selfsupporting, eliminating the need for intermediate radiator supports and for any stiffeners other than the conductive webs. Simpler and less expensive installation are two exemplary benefits that therefore result from the use of these novel conductive webs.

Another advantage of the present invention is that higher radiator temperatures may be achieved than has heretofore been possible. As a result, heat output is increased and the time required for drying and other heat treating steps may be materially shortened, increasing the speed and thereby lowering the cost of many processes. This desirable result is achieved by employing a eutectic mixture of inorganic salts or a liquid hydrocarbon having an extremely high boiling point as the circulating heat transfer medium rather than water or steam as in present radiator systems. In addition, as the heat transfer medium always remains in the liquid state, none of the problems attending the use of steam as a heat transfer medium are encountered. Moreover, system components designed to withstand lower pressures and therefore much less expensive than the components employed in the lower temperature prior art steam heated systems may be employed since the system only need be pressurized to the extent necessary to circulate the liquid through the system.

In some processes it is desirable to accomplish a heat treating step by contacting the material or area to be treated with heated air rather than radiant energy. Principles of the present invention may also be utilized to advantage to produce novel improved air heating apparatus.

As discussed in detail in my copending application No. 271,801 filed April 17, 1963, for Dryers, now US. Patent No. 3,208,158, presently available air heaters are of two general types, those in which the air is heated by diluting or mingling it with hot combustion products (direct type air heaters) and those in which the air is indirectly heated by steam. There are a number of objections to both types of prior art air heaters. In direct air heaters, the requisite combustion products are conventionally generated by one or more gas-fired burners mounted on the process machinery. These burners may pose a rather serious fire hazard. Second, the combustion products contact and may, therefore, contaminate the material being heat treated. Third, the efficiency of the burners available for this type of drier is low and their operating cost is therefore high. For particular applications, direct .air heaters may have additional disadvantages discussed in detail in the copending application referred to above.

In indirect type air heaters, the air is heated by passing it over steam heated coils. Such heaters obviate many of the objections of direct air heaters. They, however, have another serious drawback which seriously limits their utilitythe practical maximum air temperature which can be achieved in such heaters is limited to about 300-350 F. Consequently, the differential between the temperature of the air and the material being heat treated is often small and the process efliciency is low.

In general, the novel indirect type air heaters provided by the present invention include a radiator of the type provided by the present invention, and discussed above, and a plenum chamber opening onto one side of the radiator. In this application of the novel radiators of the present invention, the conductive webs disposed between adjacent tube runs are slotted to permit the passage of air therethrough. In operation, air is forced into the plenum chamber and through the slots in the webs between the tube runs. As the air wipes over the radiators radiant surfaces and passes through the web slots, it is rapidly heated to a temperature approaching that of the radiant surfaces. Two chief advantages are obtained by this novel construction. First, the air may be heated to temperatures at least as high as is possible in a direct air heater without contaminating the air with combustion products. Second, the treating .air may be heated to temperatures much higher than is possible with currently available indirect type air heaters.

It is, therefore, another object of the present invention to provide novel improved air heaters which will provide heated air uncontaminated by combustion products at temperatures higher than those practically attainable in currently available indirect type air heaters.

In many processes involving the heating of sheets or webs of material, it is essential that the sheet or web be uniformly heated across its entire width. In heating such materials with radiators of the type disclosed by the present invention, the radiator is normally spaced from and disposed parallel to the-sheet or web being heated. It will be apparent that, if the surface being heated and the radiator are the same width, the edges of the sheet receive less heat energy than areas toward the center of the sheet because some of the energy from the radiator will be directed beyond the edges of the surface being heated.

In the conventional installation, this variation in heat distribution is avoided by making the radiators substantially wider than the surface being heated so that there will be an angle of not greater than 45 degrees between the edge of the radiant surface of the radiator and the apposed edge of the surface being heated. The necessity of making the radiators wider than the surface being heated increases their initial cost and also their operational cost since more of the heat transfer medium must be circulated through them than would be necessary if they were the same width as the surface being heated.

As is disclosed in my Patent No. 3,174,228, the necessity of making the radiator wider than the surface to be heated can be avoided by employing reflectors on the edges of the radiators to reflect the energy emitted therefrom back to the surface being heated. This substantially reduces the size of the radiator required, lowering both its initial and operating cost.

The present invention, therefore, has the further advantage that a uniform distribution of radiant heat across the entire Width of a surface being heated may be obtained without making the radiator substantially wider than the surface as has heretofore been necessary.

A further object of the present invention, therefore, is the provisions of novel radiator units for distributing radiant energy at an equal flux density across the entire span of a surface to be heated.

The principles of the present invention may also be utilized to advantage in heat exchangers employed to absorb rather than radiate heat. For example, the principles of the present invention may beeflicaciously employed to provide a more eflicient tube wall for the radiant sections of steam generating and similar fluid heating units.

As discussed above, the tubular heat exchangers of the present invention, due to the combined use of an emissive coating and conductive webs, are highly efficient emitters of radiant energy. Bodies that are good emitters are equally good absorbers of radiation, and it can readily be shown that their absorptivities are equal to their emissivities. Therefore, heat exchangers employing the novel combination of conductive webs and emissive coating discussed above are highly effective absorbers of radiations and may be advantageously employed in circumstances where absorption of radiant energy is required.

Other objects of the present invention include:

(1) The provision of novel, improved radiators which may be fabricated in sizes suiting them for distributing radiant energy over larger areas than has heretofore been possible;

V (2) The provision of novel, improved radiators which are able to accommodate higher volume rates of flow of heat transfer fluid than radiators currently available and which have a greater radiant heat output per unit area of radiant surface than currently available radiators;

(3) The provision of novel, improved radiators having radiant surfaces that are more efficient emitters of radiant energy than the radiant surfaces of the radiators heretofore available;

(4) The provision of novel, improved radiators which may be fabricated in such a manner that they will emit a substantially uniform pattern of radiation over their entire radiant surface;

(5) The provision of novel, improved radiators which have radiant surfaces of substantially greater area than heretofore available radiators of comparable size;

(6) Theprovision of novel improved radiators in accordance with the foregoing objects which are inexpensively manufactured and which have a long useful life;

(7) The provision of novel, radiant heating installations employing a circulating liquid heat transfer medium and in which the radiators may be heated to higher temperatures than has heretofore been possible in systems of this type; i

(8) The provision of novel radiant heating installations having a circulating heat transfer medium which always remains in the liquid state;

(9) The provision of novel improved radiators for surface heating applications which will distribute heat evenly across the entire surface being heated and which are, nevertheless, substantially no wider than the surface; and

(10) The provision of novel improved radiators which are self-supporting, even in very large sizes, thus eliminating any need for stiffeners or intermediate supports.

Other objects and further novel features of the present invention will become more fully apparent from the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing, in which:

FIGURE 1 is a diagrammatic illustration of a novel heating system employing radiators of the type provided by the present invention;

FIGURE 2 is a front elevation of one form of radiator constructed in accordance with the principles of the present invention;

FIGURE 3 is a top plan view of the radiator of FIG- URE 2;

, FIGURE 3 is a left-hand end view of the radiator of FIGURE 2;

FIGURE 5 is asection through the radiator of FIG- URE 2, taken substantially along line 55 of the latter figure;

FIGURE 6 is a diagrammatic illustration of temperature distribution across the radiant surface of the radiator of FIGURE 2;

FIGURE 7 is an illustration similar to FIGURE 6, but shows the effect of decreasing the spacing between adjacent runs of the radiator tubes;

FIGURE 8 is a diagrammatic illustration, similar to FIGURES 6 and 7, of the temperature distribution across the radiant webs between the adjacent tube runs in radiators constructed in accordance with the principles of the present invention; 7

FIGURE 9 is a chart of the temperature distribution in uniformly sectioned webs, and in we'bs having tapered sections;

FIGURE 10 is a figure similar to FIGURE 8 of the temperature distribution across a web having a tapered section and the same area as the web illustrated in FIGURE 8;

FIGURE 11 is a chart of the temperature distribution across the web of FIGURE 10;

FIGURE 12 is a view similar to FIGURES 8 and 10, but illustrating the temperature distribution across a web having a greater cross sectional area than the webs shown in FIGURES 8 and 10;

FIGURE 13 is a graph of the temperature distribution across the web of FIGURE 12;

FIGURE 14 is a diagrammatic view of an exemplary application of the novel radiators of the present invention to the heating of webs or sheets of material;

FIGURE 15 is a graphical illustration of the distribution of heat emitted from the radiator of FIGURE 14 and impinging upon the material being heated;

FIGURE 16 is a tabulation of numerical information, part of which is shown in graphic form in FIGURE 15;

FIGURE 17 is a partial front elevation of a second form of radiator constructed in accordance with the principles of the present invention;

FIGURE 18 is a top plan view of the radiator of FIG- URE 17;

FIGURE 19 is a section through the radiator of FIG- URE 17, taken substantially along line 19--19 of the latter'figure;

FIGURE 20 is a partial front elevation of a third form of radiator constructed in accordance with the principles of the present invention;

FIGURE 21 is a top plan view of the radiator of FIG- URE 20;

FIGURE 22 is a section through the radiator of FIG- URE 20, taken substantially along line 21-21 of the latter figure;

FIGURE 23 is a partially diagrammatic view of an air heating system constructed in accordance with the principles of the present invention;

FIGURE 24 is a section through the air heater employed in the system of FIGURE 23 and is taken substantially along line 2424 of the latter figure;

FIGURE 25 is a section through the air heater, taken substantially along line 25-25 of FIGURE 23;

FIGURE 26 is a graphical illustration of the projection of radiation from point sources onto a plane surface;

FIGURE 27 is a plan view of another form of the present invention which may be made self-supporting, even in very large sizes;

FIGURE 28 is a section through the radiator of FIG- URE 27, taken substantially along line 28-48 of the latter figure;

FIGURE 29 is a view similar to FIGURE 28 of a radiator of the type illustrated in FIGURE 27, but employing a modified form of conductive web; and

FIGURE 30 is a view similar to FIGURE 29 of a radiator of the type shown in FIGURE 27, but employing yet another form of conductive Web.

Referring now to FIGURE 1 of the drawing, heating system 20 includes a fluid heating unit 22, a novel radiator 24 constructed in accordance with the principles of the present invention, a closed system of flow conduits for circulating a heat transfer medium through the system, and a pump 26 for effecting How of the heat transfer fluid through the system.

One of the novel features of the present invention resides in employing a high boiling point hydrocarbon liquid or a eutectic salt mixture as the circulating medium, permitting the medium to be circulated at extremely high temperatures in liquid form. Consequently, the heat transfer medium may be heated 'to very high temperatures and yet the heating system components need be designed to withstand only very low pressures.

The preferred heat transfer liquids include Smitherm A and Smitherm D, which are chlorinated biphenyls available from the Smitherm Division of Hupp Corporation. For temperatures up to 450500 F., Smitherm A is preferably employed. Smitherm D may be circulated at temperatures up to 750800 F. The more important physical characteristics of these heat transfer liquids are tabulated below:

"Smitherm heat transfer liquid A r Temperature Thermal Con- Enthalpy, Specific Heat, duct., B.t.u./ B.t.u.llb. B.t.u./lb., F. ftfl, hr., F./

F. C. it.

Temperature Vapor Pres, Density, 1b.] Viscosity, 1b.] mm. Hg Abgal. hr.- solute F. C.

1 Rs 1 a "Smitherm heat transfer liquid D Vapor Viscosity Density Temp, Pressure F. (mm.

Hg) (05.) (lbs./tt.-hr.) (Gms./cc.) (Lbs/gal.)

Thermal Conductivity 100 0. 350 32. 25X10- 0778 200 0. 385 31. 40 (10- 0760 300 0. 420 30. 50X10- 0732 400 0. 455 29. 65 10- 0724 500 0. 495 28. S0X10 0705 600 0. 530 0687 700 0. 565 0670 S00 0. 600 0653 1 Extrapolations.

Refractive index 1,6413 Pour point, F. +45 Flash point, F 460 Fire point, F 540 A.I.T., F. 1115 Thermal decomposition, F. 800

Electrical Properties 60 Cycle 1,000 Cycle Dielectric Constant at 100 C 3. 77 3. 54 Power Factor at 100 0., Percent.. 19. 3 1. 21 Resistivity, ohm-cm At 100 C /29 10" Oxidation-corrosion test (48 hrs., 450 F., air) Percent viscosity Incr.:

For temperatures above 800 F., HTS is preferably employed as the circulating heat transfer medium. HTS is a mixture of 40% sodium nitrite, 7% sodium nitrate, and 53% potassium nitrate (or a variation such as 55% potassium nitrate and 45% sodium nitrite). It has a freezing point of approximately 290 F. and may be used at temperatures up to 1100 F. The characteristics of HTS are described in detail in Zimmerman and Levine, Handbook of Material Trade Names (1953 edition), page 12 and in an article entitled, Molten Salt for Heat Transfer, in pages 129-135 of the May 27, 1963, issue of Chemical Engineering which are hereby incorporated in the present application by reference. When HTS is employed as the circulating medium, the novel radiators disclosed herein are preferably modified in the manner described in my copending application No. 323,840 filed November 14, 1963 for System, Apparatus, and Process.

Referring again to FIGURE 1, heating unit 22 is preferably of the shell and tube type. As illustrated, heating unit 22 includes sinuous heating tubes 28 (only one of which is shown) through which the circulating medium flows and over which hot gases generated by combustion units 30 pass. Heating tubes 28 and one or more combustion units 30 are housed in an outer shell 32 which is preferably lined with an appnopriate refractory (not shown) to radiate heat to heating tubes 28. The combustion units 30 may be either gas or oil burners or, if heating unit 22 is of larger capacity, may be coal fired.

The outlets of heating tubes 28 are connected to the main supply conduit 34 through which the heated circulating medium flows to radiator 24. From radiator 24, the circulating heat transfer medium is returned to heating unit 22 through main return conduit 36. In the illustrated diagrammatic figure, circulating pump 26 is interposed in return conduit 36-. This location is not critical, but it is merely exemplary of several possible locations for the circulating pump.

The above-described portion of the heating system is not, in itself, claimed to be novel and an elaborate description of its components is therefore not deemed necessary. One suitable heating and circulating system is that shown in copending application No. 237,817 filed November 15, 1962, by Horace L. Smith, Jr., for High Temperature Heating Apparatus to which reference may be had if deemed necessary for an understanding of the present invention.

The primary novelty of the present invention resides, in one aspect of the invention, in the novel radiator units which it provides 'and in their employment in heating systems of the general type described above.

Referring next to FIGURES 2-4 of the drawing, radiator 24 includes two sinuous, internested tube assemblies 3 8 and 40 providing labyrinthine flow paths for the heat transfer medium circulated through heating system 20 by pump 26. Tube assembly 40 is formed from a single tube bent to form parallel, spaced, side-by-side straight runs 42 connected, alternately, by end bends 44 at the left-hand end of the radiator and end bends 46 at the radiators right-hand end. Tube assembly 38, like tube assembly 40, is formed from a single tube and consists of straight runs 48 interconnected by end bends 50 at the left-hand end of the radiator and end bends 52 at the radiators right-hand end. As is best shown in FIGURE 4, the end bends 44 and 46 of tube assembly 40 are also bent outwardly to one side of the radiator and the end bends 50 and 52 of tube assembly 38 are similarly displaced toward the other side of the radiator, permitting tube assemblies 38 and 40 to be internested as shown in FIGURE 4 with the centerlines of the straight runs 42 of tube assembly 40 and the straight runs 48 of tube assembly 3'8 lying in the same plane.

Tube assembly 38 has. an inlet 54 and an outlet 56. Tube assembly 40 has an inlet 58 and an outlet 60. As is shown 'by the arrows in FIGURE 2, the heat transfer medium flows in opposite directions through the two tube assemblies 38 and 40, providing the most efficient exchange of heat between the heat transfer fluid and the tube assemblies possible.

As is best shown in FIGURES 2 and 5, rectangular webs of conductive material 62, extending substantially the length of radiator 24, are interconnected between each straight run 42 of tube assembly 40 and the adjacent straight runs 48 of tube assembly 38,-as by welding. Similar Webs 64 are fixed to the top of the uppermost tube run 48 and to the bottom of the lowermost tube run 42. Conductive webs 62 and 64 increase the radiant surface of radiator 24 and, in addition, help bring about a substantially uniform emission of radiant energy across the entire radiant surface of radiator 24, since the net effect of the internested tube assemblies, conductive webs, and the counterflow circulation of heat transfer fluid described above is to maintain the entire radiant surface of radiator 2-4 at a substantially uniform temperature.

The effect of the conductive webs can best be understood from a consideration of FIGURES 6 and 7. FIG- 4 URE 6 shows the temperature gradient in a conductive web 8 inches wide and 4 inch thick disposed between tube runs 42 and 48 maintained at a temperature of 580 F. As will be noted from this figure, the surface temperature of the web decreases from 580 F. at its juncture with tube run 42 to 467 F. at a point one-half inch from the centerline of the web. The radiant heat output decreases from 1200 B.t.u. per square foot per hour at the tubeweb juncture to 1000 B.t.u. per square foot per hour at the center of the web. A web of this configuration is, as a practical matter, not useful because of the extreme temperature gradient.

FIGURE 7 illustrates the efiect of thickening and reducing the width of web 62. In this example, the web thickness has been increased from A to of an inch and its Width reduced from 8 to 2 inches. The other assumed parameters remain the same. In this case, the temperature decreases from 80 F. at the juncture of web 62 and tube run 42 to 578 F. at the center of the Web, a decrease of only F. The radiant heat output remains substantially constant at 1200 B.t.u. per square foot per hour across the entire web.

In actual practice, assuming that tube assemblies 38 and 40 are fabricated from 2 inch pipe, web 62 would probably, in most installations, be on the order of inch thick and from 2-4 inches wide.

Referring next to FIGURE 8, it will be apparent that the temperature of web 62 is substantially lower at its midpoint than at its edges. The rapid decrease in temperature of the web from its edges to its midpoint is shown in graphical form in FIGURE 9 by curve 66.

Turning now to FIGURE 10, the web 68 interposed between tube runs 70 and 72 (which are identical to the tube runs 42 and 48 in FIGURE 8) has a double tapered cross section; i.e., both sides of web 68 have a V-like configuration with the apex of the V at the midpoint of the web. In the illustrated exemplary example, web 68 is W inch thick at its edges, inch thick at its midpoint, and has the same cross sectional area as the web 62 of FIGURE 8. distribution in web 68 is shown in graphical form by the curve 74 of FIGURE 11. Curve 74 is also shown in FIGURE 9, together with the curve 66 representing the temperature distribution in the uniformly sectioned web 62 of FIGURE 8.- It will be readily apparent, from The temperature a comparison of curves 66 and 74, that there is a substantially more nearly linear distribution of temperature in tapered web 68 than there is in the uniformly sectioned web 62.

By employing double-tapered webs in the novel radiators of the present invention, the radiant heat output per unit weight of web material is materially increased. As a result, a radiator of a given radiant heat output may be made substantially lighter; or, conversely, for a radiator of given weight, the radiant heat output may be materially increased by employing double-tapered webs.

Another advantage of the double-tapered construction is that, for a web of given cross sectional area, a web having this construction will have a substantially larger area in contact with the tube runs to which it is joined than a uniformly sectioned web of the same cross sectional area. This is readily apparent from a comparison of FIGURES 8 and 10. Since the double-tapered construction has a larger area in contact with the tube runs, heat is conducted with greater efiiciency from the heat transfer medium flowing through the tube runs into the Web, increasing the efficiency of the radiator.

The double-tapered webs illustrated in FIGURE-10 and discussed above need not necessarily be employed to obtain the foregoing advantages. These may also be obtained by employing a non-uniformly sectioned web having one flat surface like the uniformly sectioned webs 62 in FIGURE 8 and one V-like surface as is employed in the double-tapered web 68 of FIGURE 10.

As is shown in FIGURE 12, the degree to which the beneficial results discussed above may be obtained is dependent upon the degree of taper and upon the webs cross sectional area. The double-tapered Web 76 illus trated in FIGURE 12 has a somewhat greater cross sectional area than the Webs 62 and 68 in FIGURES 8 and 10 and decreases in width from 4 inch at its side to 7 of an inch at its midpoint. The curve 78 shown in FIGURES 9 and 13 represent the temperature distribution in web 76. As may best be seen by comparing curves 74 and 78 in FIGURE 9, web 76 not only has a more linear temperature distribution than web 68, but there is also a materially smaller decrease in temperature from the edges to the midpoint of the web.

Referring next to FIGURE 5, the efliciency of radiator 24 is substantially increased by enhancing the emissivity of the radiators radiant surfaces identified generally by reference characters 80 and 82. This is accomplished by coating radiant surfaces 80 and 82 with a highly emissive material. The coating may be applied in any suitable manner, as by chemical means such as anodizing, or by brushing, spraying, or rolling followed by subsequent baking or heat treatment, or by electrical deposition. Examples of suitable coatings are the colored silicone varnishes, lamp black applied in an appropriate vehicle, black enamel, lacquer and shellac.

Another highly suitable coating may be applied in accordance with the ebonizing process disclosed in United States Patent No. 2,394,899 which may be utilized to provide a smooth black oxide film or skin about 0.001 inch thick on the radiant surfaces. This black oxide film or skin is applied by cleaning the radiant surfaces, immersing the radiator in a molten bath of dichromates at a temperature of 730 F. to 750 F. for approximately 15 to 30 minutes, and then cooling and rinsing the radiator.

I have also discovered that very thin ceramic coatings such as commercially available glass frit enamels may be fused to metal surfaces to provide emissivity coefiicients up to 0.98. Such coatings are highly durable and will withstand relatively high temperatures without substantial impairment of heat transfer through the metal. Such coatings can be separated from their base, if at all, only with great difliculty, and will retain their integrity as highly emissive surface coatings for relatively long periods of time.

To apply a glass frit coating, the radiator is first cleaned by pickling or by blasting, using sand, steel shot, 1

or steel grit as an abrasive. A glass frit-water mixture having the consistency of a thick paint and called slip is then applied to the clean metal surface by spraying, flow coating, slushing or dipping. The most commonly used method is spraying. The glass slip is sprayed to a thickness of .006.010" (1.522.54 mm.) in a one coat application.

The sprayed radiator is then placed in a drier at 250- 300 F. (120150 C.). The piece is allowed to dry until all moisture has been removed. The coated panel is thus covered with very fine glass particles. This is called the bisque state. The glass in this form adheres to the piece but can be brushed off with a bristle brush or can be marred by improper handling.

The bisque coated radiator is placed in a furnace operating in the neighborhood of 1600 F. (870 C.). At this temperature the small, individual particles fuse together to form a continuous glass layer. The coated product is then removed from the furnace and allowed to cool. For most applications, one coating is all that is necessary to give good service performance. If severe corrosive conditions are to be encountered, one or more additional coats may be applied to the radiator.

The side of the radiator opposite the radiant surfaces is preferably coated with an appropriate insulating material 84 to prevent heat losses. If radiator 24 is employed in an application in which it is disposed between two areas or articles to be heated, insulation 84 is deleted and a high emissivity coating is applied to both sides of the radiator.

Radiators of the type provided by the present invention may be advantageously employed to dry or otherwise heat treat continuously moving webs of materials. In many such applications, it is necessary to ensure that the radiant energy emitted from the radiator is distributed uniformly across the entire width of the web or sheet being treated. Prior heretofore, this has been done by making the radiator substantially wider than the web or sheet being treated so that there will be an angle not greater than 45 between theplane of the radiant surface of the radiator and the line connecting the edge of the radiator and the edge of the web or sheet being treated. Substantial reductions in the width of the radiator and, therefore, in its initial and operational cost may be achieved, in accordance with the principles of the present invention, by reducing the width of the radiator to the width of the sheet or web being treated and by fixing normally extending reflectors to the edges of the radiators radiant surface.

'FIGURE 14 shows a horizontally oriented radiator 24 of the type described overlying a sheet 86 of material to be heat treated. Downwardly extending reflectors 88 and 90 are fixed to the edges of radiator 24 in any appropriate manner. The design of reflectors 88 and 90 is not critical, and any type of reflector such as polished alutninum reflectors may be employed. Reflectors 88 and 90 provide what is termed an apparent extension of the Width of radiator 24s radiant surface. In other words, the effect of employing reflectors 88 and 90 is the same as-would be achieved by increasing the width of the radiant surface of radiator 24.

The distribution of the radiant energy impinging on sheet 86 is calculated from the following basic radiant heat transfer equation:

1= 1) 2) Where: q=heat flux in B.t.u./tft. -hr. cr=Stefan-Boltzmann coefficient e=emissivity (absorptivity) of sheet 86 T =radiator surface temperature in R. T =heat sink surface temperature in R. (here the temperature of sheet 86).

F =an-gle factor for radiant energy interchange between a unit area on sheet 86 and radiator 24.

It is assumed that the emissivity of the radiator is 1.00 and that the reflectors adjacent the radiator have a reflectivity of 1.00. Values for T and T can be set by design and the value of s can be obtained experimentally in a well known manner or from handbook data. To solve the equation therefore it is only necessary to evaluate the term F for direct and reflected radiation at each point across the width of sheet 86.

The angle factor F= /2 (sin The angle factor is derived from Lamberts law of radiant energy distribution. This law may be explained as follows. Consider any small radiating surface dA. Erect a hemisphere above this surface, using the surface as its center. The law states that the intensity of radiant energy over the surface of this hemisphere varies as the cosine of the angle between the normal to the radiating surface and the line joining the radiating surface to the point of the spherical surface. That is, if dI is the intensity directly above dA, then :11, the intensity at any other point, is d1 cos 0, where 0 is the angle between the normal and the line joining dA and that point. The phrase intensity at a point refers to the rate of radiant energy reception per unit area on an area dA at the point, the area (1A being arranged perpendicular to the incident radiaation. We see that the Lambert intensity variation is equivalent to the assumption that the radiation from a surface in a direction other than normal occurs as if it came from an equivalent area having the same emissive power (per unit area) as the original surface. The equivalent area is obtained by projecting the original area upon a plane normal to the direction of radiation.

This leads to the formula dl=dl cos 0 Where all is the increment of radiation which isreceived at a point B normal to the plane of the radiator (see FIGURE 26) and d1 is the increment of radiation striking B at an angle 0 from the normal. 1

In order to derive a formula for the angle factor F involved in computing the total radiation at B due to a finite radiator it is necessary to sum the radiation from all the point sources (i.e., the entire radiator) by integrating this formula with respect to 0:

fdI=fdI cos 0, l= l f cos 0d!) 0 and 0 are extreme angles for the radiator as shown in FIGURES 8 and 20.

Splitting the integration at the normal gives 9 I=I I:L 1 cos 0d9+f 02 cos 0110] Here, the quantity inside the bracket is proportional to the angle factor. For an infinite radiator this factor must be equal to unity. Therefore:

By integrating and solving for K, we find K= /2. We then use this value in our original integration and obtain:

to the lower edge of reflector 90. Similarly, is the complement of the angle between the sheet and a line extending from point A through the lower edge of reflector 88 (the angle factor equation is also valid for installations without reflectors. In this case the lines bounding angles and are those extending to the edges of the radiant surface of the radiator). It will be apparent, therefore, that angles and depend upon the distances Z and Z from point A to the reflectors, the distance from the radiant surface of radiator 24 to sheet 86, and the width of reflectors 88 and 90. a

In the illustrated example, the radiant surface of radiator 24 is 84 inches wide, the distance from the radiant surface to sheet 86 is 6 inches, and reflectors 88 and 90 are 3 inches wide. These are typical practical dimensions for this application of the novel radiators provided by the present invention.

FIGURE 16 shows in tabular form the calculations made in deter-mining the angle factor F at various exemplary points A across the width of sheet 86. FIGURE illustrates in graphic form the variation in angle factor across the width of sheet 86. The solid line identified by reference character 92 is the angle factor without reflectors and the solid line identified by reference character 94 is the angle factor with reflectors of the dimensions described above. It will be apparent, from a comparison of these two curves, that the angle factor is materially increased, especially at the edges of sheet 86, by the use of reflectors 88 and 90.

As shown by the basic radiant heat transfer equation on column 11, the distribution of radiant energy across sheet 86, for a given set of equilibrium conditions, is linearly proportional to the angle factor. Therefore, for given equilibrium conditions, a curve of the flux distribution across sheet 86 would have the same shape as the corresponding angle factor curve. Thus, for the radiator 24 shown in FIGURE 8, both the angle factor curves and flux distribution curves would have the shape of the exemplary no reflector and partial reflector curves 92 and 94 in FIGURE 15. For the flux distribution curves, the vertical scale of FIGURE 15 would be in B.t.u./ft. -hr. or corresponding units and the illustrated curves would shift vertically as e, 0', T and T were varied. Therefore, the effect of reflectors can best be evaluated by a consideration of the angle factors calculated for a particular radiator since the angle factor is independent of operating conditions. Once the angle factor curve for a particular radiator-reflector combination is plotted, the effect of the reflectors on the flux distribution can be determined by inspection; and the numerical flux distribution values for particular operating conditions can, if desired, be'calculated by employing the basic radiant heat transfer equation.

The foregoing mathematical analysis of the effect of the reflectors is applicable, not only to the novel radiators disclosed in the present invention, but also to any radiator having a radiant surface emitting energy at substantially uniform intensity. The foregoing analysis is applicable also to reflectors varying in width from zero to a reflector substantially wide enough to touch the surface of the material being treated.

The radiator 24a shown in FIGURES 17-19 is in many respects similar to the radiator 24 shown in FIGURES 2-5. Therefore, insofar as components of the two radiators are identical, like reference characters have been employed to identify these components except that the components of radiator 244 are followed by the letter a.

The two tube assemblies 38a and 40a of radiator 24a are formed in the same manner so only the fabrication of tube assembly 38a will be discussed in detail, it being understood that these remarks apply also to tube assembly 42a.

Referring first to FIGURE 17, tube assembly 38a consists of straight runs 48a and end bends 50a which are independent members and are joined as by welding. As

v assemblies 38a and 40a to be assembled in internested rela- 138 provided with an inlet 140.

tionship with the centerlines of the straight runs 42a in tube assembly 40a and the straight runs 48a in tube assembly 38a located in the same plane.

The radiator 24b shown in FIGURES 20-22 is, in many respects, similar to the previously described embodiments. Like components have therefore been identified by like reference characters followed by the letter b.

Tube assembly 38b may besubstantially identical to tube assembly 38a. Tube assembly 401), like tube assembly 40a, consists of straight runs and end bends (50b), which are independent members. Each of the end bends 50b includes two elbows 102 and 1414 separated by a short length of tube 106 to which one end of each of the elbows is welded to form a U-shaped member. The other ends of the two elbows are welded to the juxtaposed ends of adjacent straight runs 48b.

One advantage of radiator 24b is that all of the end bends lie to one side of the radiator face 108. This feature is advantageous when it is desired to arrange the radiator in close proximity to a surface to be heated, for example.

As discussed previously, the novel features of the present invention may also be advantageously employed in the fabrication of novel improved air heaters and air heating systems. Turning now to FIGURES 23-25, air heating system 110 includes a radiator 112, a heating unit 114, a pump 116 for circulating a liquid heat transfer medium through the system, and a system of conduits indicated generally by reference character 118 forming a closed circulation path between radiator 112 and heater 114. A blower 120 is provided to circulate the air to be heated across radiator 112 and to the place of utilization.

The fluid heating and circulating system may be of the same general type described above in conjunction with the heating installation illustrated in FIGURE 1. As is also discussed in conjunction with that figure, the preferred heat transfer medium is a high boiling point hydrocarbon liquid or a eutectic salt mixture since such mediums reduce the pressure existant in the system even though they are circulated at very high temperatures.

In the illustrated air heater embodiment, radiator 112 has a single sinuous tube assembly 122 including straight, spaced apart, parallel, side-by-side straight legs 124 interconnected by end bends 126. As in the radiator embodiments described previously, conductive webs 128 are fixed to the straight legs 124 between adjacent legs. In this application of the principles of the present invention, however, webs 128 'do not extend completely from leg to leg as in the previously described embodiments, but the webs fixed to adjacent legs are sized to provide a narrow slot 130 between each pair of adjacent straight legs 124.

As is best shown in FIGURES 23 and 24, radiator 112 forms the front wall of a generally rectangularly sectioned plenum chamber 132 welded to the webs 134 and 136 fixed to the uppermost leg 124 and lowermost leg 124 of tube assembly 122. To prevent heat losses from radiator 112, the interior of plenum chamber 132 is preferably lined with appropriate insulation identified generally by reference character 144. Plenum chamber 132 terminates, at its lower end, in a transition section Inlet 140 is connected by an appropriate duct (identified generally by reference character 142) to the outletof blower 120.

In operation, the heat transfer medium is circulated by pump 116 from heating unit 114 through the tube assembly 122 of radiator 112 and returned to the heating unit. The heated circulating medium heats tube 122 and the conductive webs 128 connected to the tube legs 124 to substantially the temperature of the circulating medium.

Blower 120 forces the air to be heated through duct 142 into plenum chamber 132 and through the slots 130 between conductive Webs 128 to the area or object to be heated. As the air wipes over conductive webs 128 and the external surfaces of tube 122 and passes through slots 130, it is rapidly heated to the temperature of these surfaces.

It will also be apparent that substantial amounts of heat are radiated from the exposed face of radiator 112 identified by reference character 146. Therefore, by

positioning heating unit 114 adjacent an object or area to be heated, the object or area may be heated simultaneously by both radiant and convective heat transfer mechanisms.

Although a single tube assembly type radiator is illustrated in FIGURES 23-25, it is to be understood that any of the counterfiow type radiators previously described or, indeed, any other appropriate type of radiator may be substituted for the radiator 112 in heating unit 110. It is to be further understood that, if desired, the high emissivilty coatings discussed above may be applied to the radiant surfaces of radiator 112 and that, in such circumstances, the important advantages attendent upon the use of such coatings will be obtained.

A heat exchanger of the type shown at 112 in FIG- URES 23 and 24 can also be employed to absorb heat by utilizing it as a water wall in the radiant heating section of a steam generator, for example. In this application slots 130 may be omitted by forming webs 128 in the manner shown in FIGURE 5, for example. The side of the heat exchanger exposed to the radiant energy is provided with a coating of the kind discussed above and the remote side of the heat exchanger may be insulated as shown in FIGURE 5. Because of the high absorptivity provided by the coating and conductive Webs, however, a comparatively thin layer of insulation may be employed.

The radiator 118a illustrated in FIGURES 27 and 28 is, in most respects, similar to the radiator 112 illustrated in FIGURES 23 and 24. Like components have therefore been identified by like reference characters followed by the letter a.

Radiators 112 and 118a differ mainly in the construction of the conductive webs between the adjacent legs of the sinuous tube assembly. In radiator 118a, the conductive Webs 128a have a T cross sectional configuration, providing a rectangularly sectioned stem 148 and a doubletapered arm 150. T-section members of this configuration are widely available in a variety of sizes and materials from various producers of structural shapes.

Preferably, the T-sectioned webs 128a are so dimensioned that the neutral axes of the webs lie in the same plane as the centerlines of the tube assembly legs 124a. Thus arranged, the section modulus of the composite tube-web assembly forming radiator 118a is the sum of the section modulus of tube assembly 122a plus that of the webs 128a.

This provides a substantially higher section modulus than is obtained by using flat conductive webs such as those shown at 62 in the embodiment of FIGURE 2 for two reasons. First, the T-sectioned web 12811 has a much higher section modulus than a flat conductive web of the same width. Second, as the section moduli of the webs and the tube legs lie in the same plane, they are additive which is not the case when a flat conductive plate is employed.

Because of the substantially increased section modulus, radiators formed in the manner just described are substantially stiffer than those of the preceding embodiments; and, consequently, radiators of very long lengths which are entirely self-supporting may be readily fabricated. This, in turn, eliminates the need for auxiliary stiffening members and intermediate supports, obviating the expense and other disadvantages of such members.

Typically, tube assembly 122a may be schedule 40 tubing having an outside diameter of 1.5 inches with legs 124a spaced 4.5 inches on center. In this case standard three inch by three inch T-sectioned members (having a section modulus of 0.74) may be employed as conductive webs. The resulting composite tube-web radiator structure will have a composite section modulus of 0.887 and a weight of 25.15 pounds per square foot.

Even greater strength per unit weight of material may be obtained by employing webs having the cross sectional configuration illustrated in FIGURES 29 and 30. The web 152 illustrated in FIGURE 29 is of generally T- shaped cross sectional configuration, having a rectangularly sectioned stem 154 and arm 156. Web 152 also has, at the edge of stem 154 opposite arm 156, a generally rectangularly sectioned flange or bulb section 158. For a radiator of the dimensions discussed above, in which web 152 would have the same stem and arm dimensions as web 128a, bulb section 158 would typically be on the order of 2.0 inches wide and 0.5 inch thick. The section modulus of web 152 is'2.46 as compared to the 0.74 section modulus of web 128a. The modulus of the composite tube assembly-web structure is 2.67 (as compared to a modulus of 0.887 for the embodiment illustrated in FIGURES 27 and 28); and the radiator weight is 32.5 pounds per square foot.

It will be apparent from the foregoing that, by employing the web 152 of FIGURE 29, a much stiffer web can be provided at very little increase in weight. Conversely, in comparison with the embodiment of FIG- URE 27, an equally stiff radiator can be provided at a lower weight by employing webs having the cross sectional configuration of web 152.

The web 160 illustrated in FIGURE 30 is identical to that illustrated in FIGURE 29 except that the two web arms 162 and 164 extending from the webs stem 154 are substantially wider at the outer edges 166 where they are connected to tube legs 124a by welds 168 than at their inner edges where they are integral with stem 154. This web configuration has approximately the same section modulus as the web 152 illustrated in FIGURE 29. Where maximum efiiciency is desired, Web 160 is preferred over web 152 since the double-tapered web portion provided by flanges 162 and 164 permits maximum heat flow by conduction for the reasons discussed above in conjunction with the embodiments of FIGURES l0 and 12. However, this section is difficult and expensive to roll and may be deemed inferior to web 152, which can be readily rolled, in applications where the ultimate in etficiency is not required.

For the sake of simplicity, only a single tube run has been illustrated in FIGURE 27. It will be apparent from the foregoing, however, that the T-sectioned webs illustrated in FIGURES 27-30 may equally well be employed in radiator embodiments such as those illustrated in FIG- URE 2 in which internested tube assemblies are employed to permit counterfiow circulation.

Also, as illustrated, radiator 118a has separate end bends 170 joined to tube legs 124a as by welding. It will be apparent that tube assembly 122a could equally well be formed from a single sinuous tube as in the embodiment of FIGURE 23, if desired. Such modifications are, therefore, to be understood as being within the scope of the present invention.

Many variations in the application of the principles of this invention to tubular heat exchangers utilized to absorb heat may be made without exceeding the scope of the invention. For example, these principles may be applied to water walls composed of straight tubes and upper and lower headers as Well as to the illustrated tubular heat exchangers.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristies thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A radiant heating installation, comprising:

(a) a radiator having two sinuous internested tube assemblies providing independent flow circuits, each of said tube assemblies have substantially parallel straight runs connected by end bends, the centerlines of the straight runs of the two tube assemblies being substantially coplanar and the runs of the two tube assemblies being alternated and the end bends in one of said assemblies all lying to one side of the plane including said centerlines and the end bends in the other of said assemblies all lying to the other side of said plane; and

(b) means for heating and then subsequently effecting simultaneous counterflow of a heat transfer fluid through said independent circuits.

2. A radiant heating installation, comprising:

(a) a liquid heating unit;

(b) at least one radiator having plural independent flow circuits provided by plural sinuous internested tube assemblies each having an inlet and an Outlet;

(c) a branched supply conduit and a branched return conduit connecting said heating unit and said radiator into a closed circulation system, the branches of said supply conduit being connected in parallel to the inlets of said tube assemblies and the branches of said return conduit being connected in parallel to the outlets of the tube assemblies and the inlet of each of said tube assemblies being adjacent the outlet of another of said tube assemblies to provide counterfiow in the radiator circuits; and

(d) a high boiling point liquid heat transfer medium in said closed system.

3. The radiant heating installation as defined in claim 2, wherein said tube assemblies each include:

(a) a plurality of spaced tube runs;

(b) conductive webs extending between and substantially the length of adjacent ones of said runs; and

(c) a highly emissive and absorptive ceramic coating having a thickness on the order of 0003-0007 inch on one side of the assembly formed by said tube runs and said conductive webs.

4. A radiant heating installation, comprising:

(a) a radiator having two sinuous internested tube assemblies providing independent flow circuits, each of said tube assemblies being fabricated of a heat conductive material and having substantially parallel straight runs connected by end bends with the cen terlines of the straight runs of the two tube assemblies being substantially coplanar and conductive webs between adjacent ones of said runs, each of said webs being a separate structural member connected to two tube runs only and subtending only a minor portion of the two tube runs to which it is connected, said webs being connected directly to said tube runs and extending substantially the length thereof to increase the radiant surface of said radiator;

(b) means for heating a heat transrer fluid; and

() means for effecting simultaneous counterflow of the heated heat transfer fluid through said independent circuits.

5. The radiant heating installation as defined in claim 1, together with conductive webs extending between and substantially the length of adjacent ones of said runs, said conductive webs having a tapered cross section and being thinner at their midpoints than at their edges.

6. The radiant heating installation as claimed in claim 1, together with conductive webs having a rectangular cross section extending between and substantially the length of adjacent ones of said runs.

7. The radiant heating installation as defined in claim 1, wherein adjacent end bends lie on opposite sides of the plane including the centerlines of the straight runs.

8. The radiant heating installation as defined in claim 1, wherein each if said end bends consists of three elbows.

9. The radiant heating installation as defined in claim 1, including a coating of high emissivity material on at least one side of said radiator.

10. The radiant heating installation as defined in claim 9, wherein said coating is a ceramic material having a thickness on the order of 0.003-0.007 inch fused to the radiating surfaces of said radiator.

11. The radiant heating installation as defined in claim 1, including:

(a) a fluid heater;

(b) a supply line connected between said heater and the inlets of said tube assemblies;

(c) a return line communicating with the outlets from said tube assemblies and the heater; and

(d) a pump for forcing the heat transfer liquid seriatim through said heater and said tube assemblies.

12. The radiant heating installation as defined in claim 2, wherein each of said tube assemblies is a single tube.

13. The radiant heating installation as defined in claim 1, including:

(a) a layer of insulation on one side of said radiator;

and

(b) a coating of high emissivity material on the other side of said radiator.

14. The heating installation as defined in claim 2, wherein the decomposition rate of said liquid at 550 F. does not exceed about 0.001% by volume per hour of system operation.

15. The heating installation as defined in claim 2, wherein the decomposition rate of said liquid at 700 F. does not exceed about 0.001% by volume per hour of system operation.

16. The heating installation as defined in claim 2, wherein said liquid is selected from the group consisting of chlorinated biphenyls, polyphenyl alkyls, aryloryloxysilanes, and eutectic salts.

17. The radiant heating installation as defined in claim 5, wherein said transverse sections are symmetrical.

References Cited by the Examiner UNITED STATES PATENTS ROBERT A. OLEARY, Primary Examiner.

N. R. WILSON, Assistant Examiner. 

1. A RADIANT HEATING INSTALLATION, COMPRISING: (A) A RADIATOR HAVING TWO SINUOUS INTERNESTED TUBE ASSEMBLIES PROVIDING INDEPENDENT FLOW CIRCUITS, EACH OF SAID TUBE ASSEMBLIES HAVE SUBSTANTIALLY PARALLEL STRAIGHT RUNS CONNECTED BY END BENDS, THE CENTERLINES OF THE STRAIGHT RUNS OF THE TWO TUBE ASSEMBLIES BEING SUBSTANTIALLY COPLANAR AND THE RUNS OF THE TWO TUBE ASSEMBLIES BEING ALTERNATED AND THE END BENDS IN ONE OF SAID ASSEMBLIES ALL LYING TO ONE SIDE OF THE PLANE INCLUDING SAID CENTERLINES AND THE END BENDS IN THE OTHER OF SAID ASSMBLIES ALL LYING TO THE OTHER SIDE OF SAID PLANE; AND (B) MEANS FOR HEATING AND THEN SUBSEQUENTLY EFFECTING SIMULTANEOUS COUNTERFLOW OF A HEAT TRANSFER FLUID THROUGH SAID INDEPENDENT CIRCUITS. 